(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method for using a seal ring for a Phase Shift Mask in such a manner that no side lobe ringing problems occur.
(2) Description of the Prior Art
A photolithographic mask provides the ability to selectively expose the surface of a layer of light sensitive material for the creation of semiconductor patterns therein. The photolithographic mask contains a pattern of device features that are transposed from the mask to underlying layers of photosensitive semiconductor material such as a layer of photoresist. A standard photo mask contains a transparent substrate, typically made of quartz. A patterned layer of opaque material is created over a surface of the transparent substrate. Chromium is typically used for the opaque material, deposited to a thickness of about 1,000 Angstrom. Alternate opaque materials for the creation of the patterned layer over the surface of a photolithographic mask are nickel and aluminum.
The principle of phase shifting of the light as the light passes through the mask is widely applied in the creation of photolithographic masks. Phase shifting masks are used to create device features of sub-micron dimensions. It is well known in the art that adjacent light beams, which are in extreme close proximity to each other while the light beams are used to create sub-micron devices features, mutually influence each other, having a detrimental effect on the definition of the exposed patter. The phase shift mask counteracts this mutual influence that closely spaced light beams have on each other. As a further advance, alternate phase shifting masks are used, where the phase shifting characteristic of the phase shifting mask is alternately counteracted in the light as the light passes through the photo mask.
In the creation of photolithographic mask difficulties are encountered by the transition or boundaries between light transmitting and light blocking surface areas of the mask. Light that passes close to these boundaries is diffracted, resulting in loss of sharpness of definition of the exposed and therefrom created pattern. Ideally, the exposing light passes through the transparent surface of the mask in an equal amount of energy, even where the boundary of the transparent surface of the mask is approached. The opposite is ideally true of the opaque surface area of the mask: the light should ideally be completely blocked from passing through the surface area of the mask that is covered with opaque material, also independent of the boundary of the opaque surface area being approached. In actual implementation however light will be diffracted due to the existence of the boundary between the transparent and the opaque surface areas of the mask, a defraction that reduces definition of the created pattern. Light defraction that occurs at boundaries between transparent and opaque surface areas of a photolithographic mask is generally referred to as side lobe ringing of the transmitted light.
To compensate for effects of light defraction, methods have been provided for the compensation thereof by for instance, U.S. Pat. No. 5,795,682 (Garza), the creation of guard rings which provide compensating light transmissive regions that are located such that side lobe ringing is most effectively counteracted. These compensating light transmissive regions do not contribute to the definition of a pattern in the exposed surface. The effectiveness of the compensating transmissive regions is assured due to the fact that the radiation that causes the side lobe ringing is about 180 degrees out of phase with the radiation that passes through the transmissive regions.
The invention does not address the creation of guard rings as these guard rings have been provided by U.S. Pat. No. 5,795,682 (Garza). In the creation of a PSM, a seal ring image is provided over the surface of the mask. This seal ring image is projected onto the exposed surface, surrounding the active surface area of the exposed chip for the protection of the chip during the process of sawing or singulating the chip. This seal ring provided in or over the surface of the chip is typically a relatively wide line of metal, comprising overlaying layers of metal-1 through metal-n, that are interconnected with vias. This seal ring serves as a barrier and provides protection against mostly moisture penetration into the chip.
The instant invention addresses the creation of the seal ring image, also referred to as a fuse guard ring, over the surface of the photolithographic mask.
U.S. Pat. No. 5,795,682 (Garza) shows a guard ring to compensate for sidelobe ringing in an APSM.
U.S. Pat. No. 6,355,503 B2 (Schroeder) shows square contact holes utilizing a sidelobe formation.
U.S. Pat. No. 6,291,113 B1 (Spence) shows a side lobe suppressing PSM.
U.S. Pat. No. 6,210,841 B1 (Lin et al.) is a related APSM patent.